In order to develop insensitive munitions, it is advantageous to synthesize nano-sized energetic crystalline particles such as RDX. Parameters, which could affect the thermal and shock sensitivity of RDX-based energetic materials, include the size and shape of crystalline particles, particle size distribution, and any defects and impurities in individual crystals. In this investigation, the method utilized to produce these nano-scale energetic particles is the rapid expansion of a supercritical solution (RESS) process. Using varied operating conditions (pressure and temperature) and different nozzle throat diameters of the expansion nozzles, the dependency of particle size on these parameters has been determined. Particles ranging from 65 to 105 nm have been obtained from this RESS process using a controlled pre-expansion pressure up to 120 MPa (17,500 psi) and a controlled pre-expansion temperature up to 363 K. The synthesized particles have been analyzed using field emission scanning electron microscopy (FESEM) to obtain surface characteristics and particle size distributions. Preliminary data showed the trend of smaller synthesized particles for higher pre-expansion temperature operating conditions. This is due to the increase in solubility and higher rate of spontaneous homogeneous nucleation in the formation of nano-sized particles.